The pancreas is a complex organ consisting of distinct endocrine, exocrine and duct cell compartments that function together to regulate aspects of digestion and glucose homeostasis. Alterations in pancreas function lead to several critical human diseases, including diabetes, pancreatitis and pancreatic cancer. Despite this clear medical importance, our knowledge of how pancreatic cell lineages are specified and maintained remains incomplete. Studies on Mistl, a novel basic helix-loop-helix (bHLH) transcription factor, have provided the first insight into the molecular mechanisms responsible for the terminal differentiation and cellular identity of one pancreas lineage - the exocrine acinar cells. Mistl null (MistlK0) mice develop a defective exocrine pancreas that exhibits an increase in cell numbers, altered patterns of gene expression, defective intercellular communication, disruption of intracellular organization and acinar-to-ductal cell conversion, all of which are characteristics associated with pancreatitis and pancreatic cancer. Our working hypothesis is that Mistl functions to initiate and maintain normal epithelial cell polarity, acinar cell function and cellular identity. Experiments proposed in this application will test this hypothesis and generate information that will position Mistl within the transcription factor hierarchy controlling pancreas development, function and maintenance. Specifically, we will (i) identify how Mistl controls the developmental expansion and maturation of the exocrine pancreas and test if adenovirus-mediated, or inducible Mistl expression strategies, can rescue the Mist1KO phenotype;(ii) define the functional Mistl transcription complex(es) in pancreatic acinar cells and identify Mistl target genes;and (Hi) generate Mistl- Cre mouse lines to identify and isolate Mistl expressing cell descendants for progenitor cell molecular profiling studies. Successful completion of these studies will provide important new insights into how the bHLH transcriptional network directs pancreatic cell lineage specification, maintains lineage identity and insures normal epithelial cell organization.